The present invention relates to a developing apparatus for depositing a developing agent on an image carrier with a latent image thereon, thereby developing the latent image.
Development is achieved, for example, when a developing agent (colored powder called toner) which is charged with a polarity opposite to that of the electric charges forming an electrostatic latent image is electrostatically attracted to the electrostatic latent image. Developing agents include one-component developing agents which consist of a powdery toner only or a toner coated with SiO.sub.2 or another additive, and two-component developing agents which are composed of a powdery toner and a carrier as it is called, such as magnetic powder or fine resin powder, glass, etc. In the two-component developing agents, the toner is securely charged by friction with the carrier. To maintain a constant developing density, on the other hand, the mixture ratio between toner and carrier, i.e., toner density, must be kept constant. Requiring no such control of toner density, the one-component developing agents surpasses the two-component developing agents in easy handling.
The one-component developing agents are classified into two types, magnetic and nonmagnetic. In general, a nonmagnetic developing agent is prepared by mixing resin powder with a color agent such as carbon, while a magnetic developing agent is a mixture of resin powder and magnetic powder.
A prior art method using a one-component developing agent is an application of the so-called no-contact developing process stated in Japanese Patent Publication No. 9475/66. In this developing process, an image carrier is opposed to a layer of a developing agent on a developing agent carrier at a fixed space, and a suitable bias is applied to the developing agent layer to fly the developing agent to image portions of an electrostatic latent image on the image carrier. The no-contact developing process is superior to any other developing methods in the following points. Since a developing agent with electric insulating property or high resistance can be used in the process, there is no possibility of defective transfer. Moreover, the developing agent will not cause fog, since it will not be flown to the no-image portions of the electrostatic latent image. In developing an electrostatic latent image by the no-contact developing process, the distance between the image carrier and the developing agent carrier must be minimized for a visible image of higher quality. Naturally, therefore, the developing agent layer on the developing agent carrier needs to be very thin and uniform.
In order to form such a thin layer of developing agent, a film forming method is disclosed in Japanese Patent Disclosure No. 43037 in which a thin layer of a magnetic developing agent is formed on a developing agent carrier containing magnetic field generating means therein. According to this method, a uniformly thin layer of magnetic developing agent can be formed with high reliability. Thus, a satisfactory visible image may be obtained by the use of the no-contact developing process.
The no-contact developing process, however, requires as indispensable requisites a magnetic field generating means, i.e., a magnet, and a magnetic developing agent composed of toner and magnetic powder dispersed therein. Thus, this developing process has the following drawbacks:
(1) the use of the magnet in the developing agent carrier renders the apparatus complicated and expensive, constituting a hindrance to the reduction of the size and weight of the apparatus,
(2) the magentic developing agent is more expensive than the nonmagnetic one, and
(3) containing magnetic powder, the magnetic developing agent is poor in coloring capability and is unsuited for color print.
Thus, the no-contact developing process using the magnetic developing agent has the substantial drawbacks attributed to the use of the magnetic developing agent, as well as many advantages.
Meanwhile, a no-contact developing process using a nonmagnetic developing agent may be considered an ideal developing method which can settle all the problems related to the prior art method. However, this alternative process has one major problem in that the use of the nonmagnetic developing agent makes it difficult to form a uniformly thin layer of the developing agent stably on the developing agent carrier. Therefore, this method has not yet been put to practical use. If the thin layer of the developing agent is not uniform, the amount of the developing agent flown to the electrostatic latent image is rendered partially uneven, preventing the formation of a good-quality visible image (image formed by flying the developing agent to an electrostatic latent image).
In order to form a thin layer of the nonmagnetic developing agent, an elastic blade is pressed against the surface of the developing agent carrier. However, in this conventional developing apparatus, in a non-developing period such as a non-operative period of the apparatus, the elastic blade is kept pressed against the surface of the developing agent carrier. For this reason, the developing agent portion between the elastic blade and the developing agent carrier is kept pressed. When the developing agent is kept under pressure for an extended period of time, it becomes attached to the surface of the developing agent carrier or that of the elastic blade or particles of the developing agent cohere. Thereafter, a uniformly thin layer of developing agent cannot be formed on the surface of the developing agent carrier. This problem is easily caused under relatively high temperatures. When the developing agent used is a developing agent to be fixed under pressure, this problem is enhanced due to its good sensitivity to pressure.